This invention relates to a weighing apparatus and, more particularly, to a weighing apparatus which incorporates a load cell of the type having strain gauges affixed thereto.
Load cells of the so-called bending beam-type may be employed as weight sensors for converting the magnitude of an applied load into an electrical signal. A load cell of such type includes a stationary mounting portion, a portion for receiving the applied load, and a pair of parallel, flexible beams which connect the mounting and load-receiving portions to each other. Each beam is reduced in thickness at two separate points, and has a strain gauge affixed thereto at each such point, for a total of four strain gauges in the load cell. The four strain gauges are connected in the form of a bridge circuit, with each gauge arranged in one arm of the bridge. A constant voltage is applied across one pair of opposing junctions through a resistor, and an electric signal corresponding to the magnitude of the load applied to the load cell is obtained from across the other pair of opposing junctions. Thus, when a load is applied to the load-receiving portion of the load cell with a constant voltage applied across one pair of opposing junctions, a potential difference develops across the other pair of opposing junctions owing to a change in the resistance of each strain gauge which measures the strain developed in the beams of the load cell, the strain corresponding to the magnitude of the applied load. Sensing the potential difference makes it possible to detect the magnitude of the load.
It is conventional practice to install a load cell of the above type in a weighing apparatus in an exposed condition. As a result, oxides tend to form on the strain gauges owing to oxidation caused by water droplets and other sources of moisture. Chlorides also are likely to form on the strain gauges. Since a result of the oxidation and the chloride formation is a change in the resistance of the strain gauges, correct weight measurement cannot be performed. In addition, external moisture and contaminants in the atmosphere can also degrade and corrode the strain gauges which, with time, may no longer be able to function properly. The problem is particularly acute in weighing machines having load cells, if these machines must be washed with water periodically. The load cells in such machines are wet quite often and quickly deteriorate both physically and electrically.
Thus it has not been possible to use the conventional load cell effectively in locations where the humidity is high, the atmosphere is contaminated, or in machines that must be washed with water.
Various methods for protecting strain gauges have been proposed, one of which is coating the strain gauges with a resin and another is disposing a load cell in a gas sealed within a closed vessel. However, as to a former, the resin which has adequate dampproof- and waterproof-characteristic cannot be found. And as to the latter, there is the problem of gas-leakage and a structure which can prevent the gas from leaking is complicated.